Torque transmitting systems are widely employed in automobile transmissions selectively to control relative rotation between components of the transmission. As is well known to the art, one widely accepted form of an automatic, vehicular transmission employs compound planetary gear sets that utilize three clutch assemblies and two braking bands to determine relative rotation between selected components in order to obtain the desired function of the compound planetary gear sets. The operator selects the driving range from the neutral, forward (either the standard drive, the "Intermediate" or the "Lo" forward selections) or reverse, and the transmission automatically changes gear ratios in relation to the vehicle speed and the engine torque input, as permitted within the driving range selected. Vehicle speed and engine torque signals are constantly fed to the transmission in a manner well known to the art in order to provide the proper gear ratio for maximum efficiency and performance at all throttle openings.
A planetary gear train consists of a center, or sun, gear, an internal gear and a planetary carrier assembly which includes and supports the smaller planet gears, or pinions. When the sun gear is held stationary and power is applied to the internal gear, the planetary gears rotate in response to the power applied to the internal gear and thus "walk" circumferentially about the fixed sun gear to effect rotation of the carrier assembly in the same direction as the direction in which the internal gear is being rotated.
When any two members of the planetary gear train rotate in the same direction and at the same speed, the third member is forced to turn at the same speed. For example, when the sun gear and the internal gear rotate in the same direction, and at the same speed, the planet gears do not rotate about their own axes but rather act as wedges to lock the entire unit together to effect what is known as direct drive.
Whenever the carrier assembly is restrained from spinning freely, and power is applied to either the sun gear or the internal gear, the planet gears act as idlers. In that way the driven member is rotated in the opposite direction as the drive member. Thus, when the reverse drive range is selected, a brake band assembly is actuated frictionally to engage the carrier assembly, and restrain it against rotation, so that torque applied to the sun gear will turn the internal gear in the opposite direction in order to reverse the rotational direction of the drive wheels, and thereby reverse the direction of the vehicle itself. The friction band assemblies are normally operated by servo mechanisms, many varieties of which are known to the art, but the present invention does not relate servo mechanisms, and they will not be further described herein.
It should be appreciated that a second friction applying band assembly may also be employed when the engine compression, acting through the transmission, is being employed to effect a braking action. To understand this operation it is desirable to know that in a compound planetary gear set, multiple planetary gear sets may be employed, and adjacent planetary gear sets may utilize sun gears fabricated in one piece. A sprag assembly is frequently employed selectively to preclude the common sun gears from rotating in one direction.
When multiple planetary gear sets are employed, adjacent planetary gear sets are generally connected. Typically, the carrier of the first set is secured to the internal gear of the second set. To make the two planetary gear sets effective, a roller clutch assembly is generally employed to hold the carrier of the second set against rotation in at least one direction.
To provide a means for connecting and disconnecting the power output of a torque converter to the transmission gear train, a clutch assembly is generally employed. Normally, the clutch assembly includes a clutch housing which is splined to the input shaft. A series of torque plates are connected, as by tangs, to the clutch housing, and a second series of torque plates are connected, also by tangs, to a clutch hub member. An actuating piston is hydraulically operated frictionally to lock the torque plates together, and a release spring is employed to retract the piston when the hydraulic pressure is released. By effecting a spline connection between the main transmission shaft and the clutch hub member, whenever hydraulic pressure is supplied to the clutch assembly the input shaft directly rotates the main transmission shaft. When the hydraulic pressure is released, the clutch assembly disengages the aforesaid drive connection, and the transmission is in neutral.
A similar clutch arrangement may also be employed selectively to connect the outer race of the sprag to the transmission housing. When the outer race of the sprag is so connected to the housing, the sprag is effective in securing the sun gear connected to the sprag against rotation, and the power output from the converter is received by the transmission output shaft at the gear reduction ratio associated with "second" gear.
A third such clutch arrangement is employed to lock the pinions of the adjacent planetary gear set together so that they act as wedges to allow the two adjacent planetary gear sets to rotate as one unit. In this arrangement the power output from the converter is received by the transmission output shaft in what is designated as "third" gear.
Actuation of the first and third described clutch arrangements is generally effected when the operator selects the "reverse" range of operation.
Accordingly, it is apparent that the piston assemblies which actuate the aforesaid clutch arrangements are of significant importance to the operation of such a transmission. However, unless expensive and time consuming machining operations have been employed in making both the piston chamber and the piston itself, considerable problems have been encountered by the use of individual seals which have either been independently mounted on the piston itself or in recesses provided in the wall of the chamber in which the piston reciprocates.
In an attempt to reduce the expense required to provide a satisfactory seal between the actuating piston and the piston chamber, considerable attempts have been made to bond the seals directly to the piston, but the resulting one piece piston and seal assemblies have also had significant problems.
For example, a bonding agent has generally been employed to effect adhesion between the piston and the seal in an attempt to maintain the seal in its proper disposition relative not only to the piston but also the piston chamber. However, such bonding agents generally require a rather high temperature to be activated. For that reason the mass of the metallic piston in relation to the mass of the seal becomes significant. Specifically, the considerably greater mass of the piston requires additional time and energy to raise the temperature of the composite assembly to the temperature required to activate the bonding agent.
Attempts to reduce the mass of the piston relative to the mass of the seal have heretofore severely impaired the structural support necessary to assure that the seal remains fully operable. Experience reveals that unless the seal is sufficiently supported across the gap between the piston and the piston chamber, a surge of the pressure within the piston chamber can "blow by" the seal and destroy the effectiveness of the actuating piston assembly.